Engine systems on vehicles, such as hybrid electric vehicles (HEV) and vehicles configured for idle-stop operations, may be configured with a laser ignition system. The laser ignition system includes a laser coupled to each combustion chamber for igniting fuel. In addition to initiating cylinder combustion, the laser ignition system may also be used during engine starting to accurately determine the position of a piston in each cylinder. Laser ignition systems may provide various advantages over spark plugs which tend to degrade over time due to chemical changes at the cathode/anode and accumulation of particulate matter.
Laser ignition systems may also be used for misfire detection. One example approach is shown by Martin et al. in U.S. 20160040644. Therein, after a laser ignition system is used to ignite an air-fuel mixture in a cylinder, a temperature profile of the cylinder is sensed over the combustion event via an infra-red sensor. A misfire event is then determined based on the generated temperature profile relative to an expected profile.
However, the inventors herein have identified a potential issue with such an approach. A misfire may be identified when the cylinder does not produce sufficient torque. There may be various reasons for a cylinder to misfire, and a controller may perform different misfire mitigating actions based on the nature of the misfire. In the approach of Martin, if the laser device of the laser ignition system is degraded, the cylinder will also not produce any torque on a combustion event, generating a misfire. Consequently, the controller may be confounded as to the reason for the misfire. For example, it may be difficult for the controller to determine if the misfire was due to laser ignition system degradation, an air-fuel mixture having a richer than intended (e.g., richer than stoichiometric) air-fuel ratio, weak cylinder combustion (due to leaky intake or exhaust valves, for example), a leaky/plugged fuel injector, moisture/humidity in the cylinder, bad fuel quality, hot cylinder walls, leaky canister purge valve, excess EGR flow, etc.
In one example, the above issue may be addressed by a method for diagnosing the laser ignition system so as to differentiate laser degradation induced cylinder misfire from other misfire causes. One example method includes spinning an engine in reverse, unfueled, to establish a baseline intake air temperature and then sealing a cylinder at a position of negative valve overlap; operating a laser ignition device in the sealed cylinder; spinning the engine in reverse, unfueled, to unseal the cylinder; and diagnosing the laser ignition device based on a change in measured intake air temperature relative to the baseline intake temperature. In this way, a laser ignition system may be more robustly diagnosed while minimizing noise factors.
As an example, after an engine soak to ambient temperature following a key-off, an engine controller (e.g., an engine's powertrain control module or PCM) may wake up to diagnose the laser ignition system. The controller may spin the engine unfueled in a reverse direction (that is, opposite to the direction the engine is spun in during cranking) for a duration (e.g., for 15 seconds). By spinning the engine in reverse, an unfueled, un-combusted intake air stream is established with all cylinder laser igniters deactivated, and a corresponding baseline intake temperature for the (unfueled and un-combusted) intake stream may be noted. For example, the baseline intake air stream temperature may be measured by an intake air temperature sensor coupled to the intake passage. Once the baseline intake temperature is established, the controller may spin the engine slowly to park the engine in a position where a first cylinder, selected for laser ignition diagnostics, is sealed. Specifically, at the sealed position, both the intake and exhaust valves of the selected first cylinder may be fully closed, such as at a position of negative intake to exhaust valve overlap. The laser of the sealed cylinder is then activated, which causes heat to be generated in the cylinder (since there is no fuel to combust), and then trapped therein due to the valves being closed. After a duration of laser operation (e.g., a few minutes), the laser is deactivated and the engine is spun, unfueled, and in reverse to a position where at least the intake valve is open and the heated air from the cylinder is released into the intake passage. Optionally, if the laser is maneuverable, the laser beam from the igniter may be focused on an area near the intake valve to achieve more localized heat as that is the exit point for air out of the cylinder. Otherwise, the laser heats the top of the piston. A subsequent rise in the intake temperature (relative to the baseline value) following the engine reverse spinning indicates that the laser coupled to the first cylinder is functioning. Else, if the temperature does not rise, it may be determined that the laser coupled to the first cylinder is not functioning and a diagnostic code with a unique identifier for the first cylinder may be set. The controller may then proceed to re-establish a baseline intake temperature and similarly assess remaining cylinders, one at a time, by sealing the cylinder, operating the laser ignition device in the sealed cylinder, and spinning the engine, unfueled, in reverse. In still further examples, the engine may be subsequently spun forward and an exhaust temperature sensor may be used to diagnose the laser. In still a further example, the engine may be coupled in a vehicle configured with autonomous driving capabilities, and the diagnostic may be performed while the vehicle is in an autonomous mode.
In this way, the ability of laser ignitors to heat a metallic object upon impingement of the laser beam can be advantageously leveraged to diagnose the laser coupled to each cylinder. The technical effect of sealing a cylinder by reverse spinning the engine to a position where cylinder valves are closed, and operating the laser in the sealed cylinder, is that a temperature inside the cylinder can be raised. By then reverse spinning the engine to open the valves, the additional heat can be transferred to an intake stream. By comparing the change in intake air temperature following the opening of the cylinder valves relative to a previously established baseline temperature, the heat transfer from the laser ignition device can be measured, and the functionality of the laser can be inferred. By relying on the laser's motive energy and resultant heat generation during engine-off conditions, the laser can be diagnosed robustly and without confounding the results with noise factors from engine combustion. By performing the diagnostic sequentially on all engine cylinders, individual cylinder laser systems can be accurately diagnosed. By enabling a laser ignition system to be diagnosed, engine misfires due to ineffective laser operation in an engine system configured with laser ignition can be better distinguished from other misfire causes. Consequently, engine misfires may be mitigated in a timely and appropriate manner.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.